1. Field of the Invention
The subject invention is generally related to systems for delivering drilling fluid (or “drilling mud”) for oil and gas drilling applications and is specifically directed to a method and apparatus for varying the density of drilling mud in deep water oil and gas drilling applications.
2. Description of the Prior Art
It is well known to use drilling mud to provide hydraulic horse power for operating drill bits, to maintain hydrostatic pressure, to cool the wellbore during drilling operations, and to carry away particulate matter when drilling for oil and gas in subterranean wells. In conventional drilling operations, a well is drilled using a drill bit mounted on the end of a drill stem inserted down the drill pipe. The drilling mud is pumped down the drill pipe to provide the hydraulic horsepower necessary to operate the drill bit. A gas flow and/or other additives are also pumped into the drill pipe to control the density of the mud. The mud passes through the drill bit and flows upwardly along the periphery of the drill string inside the open hole and casing, carrying particles loosened by the drill bit to the surface. At the surface, the return mud is cleaned to remove the particles and then is recycled down into the hole. In basic operations, drilling mud is pumped down the drill pipe to provide the hydraulic horsepower necessary to operate the drill bit, and then it flows back up from the drill bit along the periphery of the drill pipe and inside the open borehole and casing. The returning mud carries the particles loosed by the drill bit (i.e., “drill cuttings”) to the surface. At the surface, the return mud is cleaned to remove the particles and then is recycled down into the hole.
In other non-conventional drilling operations, such as drilling with casing operations, the hole is drilled not with a typical drill bit, but rather with a bottom hole assembly which is run on a drill string through the casing to facilitate drilling of the borehole. Alternatively, a drillable bottom hole assembly may be mounted to the bottom of the casing and the entire casing may be rotated at the surface to facilitate drilling of the borehole. The advantage of drilling with casing is that the well can be drilled, cased, and cemented during one downhole trip, as opposed to drilling the borehole, retrieving the drill bit, and then running and cementing the casing downhole. Examples of drilling with casing systems includes Tesco Corporation's Casing Drilling™ system and Weatherford's Drillshoe™ system.
In both conventional and non-conventional drilling application, a mud management system must be employed to monitor and control the density of the drilling mud in order to maximize the efficiency of the drilling operation and to maintain the hydrostatic pressure. One example of such a system is shown and described in U.S. Pat. No. 5,873,420, entitled: “Air and Mud Control System for Underbalanced Drilling”, issued on Feb. 23, 1999 to Marvin Gearhart. The system shown and described in the Gearhart patent provides for a gas flow in the tubing for mixing the gas with the mud in a desired ratio so that the mud density is reduced to permit enhanced drilling rates by maintaining the well in an underbalanced condition.
It is known that there is a preexistent pressure on the formations of the earth, which, in general, increases as a function of depth due to the weight of the overburden on particular strata. This weight increases with depth so the prevailing or quiescent bottom hole pressure is increased in a generally linear curve with respect to depth. As the well depth is doubled in a normal-pressured formation, the pressure is likewise doubled. This is further complicated when drilling in deep water or ultra deep water because of the pressure on the sea floor by the water above it. Thus, high pressure conditions exist at the beginning of the hole and increase as the well is drilled. It is important to maintain a balance between the mud density and pressure and the hole pressure. Otherwise, the pressure in the hole will force material back into the wellbore and cause what is commonly known as a “kick.” In basic terms, a kick occurs when the gases or fluids in the wellbore flow out of the formation into the wellbore and bubble upward. When the standing column of drilling fluid is equal to or greater than the pressure at the depth of the borehole, the conditions leading to a kick are minimized. When the mud density is insufficient, the gases or fluids in the borehole can cause the mud to decrease in density and become so light that a kick occurs.
Kicks are a threat to drilling operations and a significant risk to both drilling personnel and the environment. Typically blowout preventers (or “BOP's”) are installed at the ocean floor or at the surface to contain the wellbore and to prevent a kick from becoming a “blowout” where the gases or fluids in the wellbore overcome the BOP and flow upward creating an out-of-balance well condition. However, the primary method for minimizing the risk of a blowout condition is the proper balancing of the drilling mud density to maintain the well in a balanced condition at all times. While BOP's can contain a kick and prevent a blowout from occurring thereby minimizing the damage to personnel and the environment, the well is usually lost once a kick occurs, even if contained. It is far more efficient and desirable to use proper mud control techniques in order to reduce the risk of a kick than it is to contain a kick once it occurs.
In order to maintain a safe margin, the column of drilling mud in the annular space around the drill stem is of sufficient weight and density to produce a high enough pressure to limit risk to near-zero in normal drilling conditions. While this is desirable, it unfortunately slows down the drilling process. In some cases underbalanced drilling has been attempted in order to increase the drilling rate. However, to the present day, the mud density is the main component for maintaining a pressurized well under control.
Deep water and ultra deep water drilling has its own set of problems coupled with the need to provide a high density drilling mud in a wellbore that starts several thousand feet below sea level. The pressure at the beginning of the hole is equal to the hydrostatic pressure of the seawater above it, but the mud must travel from the sea surface to the sea floor before its density is useful. It is well recognized that it would be desirable to maintain mud density at or near seawater density (or 8.6 PPG) when above the borehole and at a heavier density from the seabed down into the well. In the past, pumps have been employed near the seabed for pumping out the returning mud and cuttings from the seabed above the BOP's and to the surface using a return line that is separate from the riser. This system is expensive to install, as it requires separate lines, expensive to maintain, and very expensive to run. Another experimental method employs the injection of low density particles—such—as glass beads into the returning fluid in the riser above the sea floor to reduce the density of the returning mud as it is brought to the surface. Typically, the BOP stack is on the sea floor and the glass beads are injected above the BOP stack.
While it has been proven desirable to reduce drilling mud density at a location near and below the seabed in a wellbore, there are no prior art techniques that effectively accomplish this objective.